The way a crow shook down on me is the first line of the poem Dust of Snow written by Robert Frost. The poet says that he is on a way i.e. outside (might be wandering alone to enjoy the snow). Suddenly a crow, which is on the branch of a tree shakes making the snow fall on the poet.
As these are the initial lines of the poem, they make the reader eager to know what happens next because crow is considered to be a bad-omen in the country of the poet.Read the detailed summary of the poem here.